1. Field of the Invention
The invention relates generally to retrofittable lamps configured as standard incandescent lamps but with LED illumination soruces, the lamps of the invention being capable of substitution in illuminated signs such as exit signs and the like for previously used incandescent lamps.
2. Description of the Prior Art
It is estimated that approximately 40 million illuminated exit signs exist in operating capacities in this country at the present time. The great majority of this exit signage have as the illumination source one or more incandescent lamps. Millions of other illuminated signs also use incandescent lamps as the light source. While the incandescent lamps used in these illuminated signs have an exceptionally long life for incandescent lamps, a problem which exists with these lamps is the fact that the lamps burn out over time and must be replaced. Replacement of a burned out incandescent lamp in an illuminated sign with an essentially identical incandescent lamp is the common practice even though the necessity for replacement will again exist within a relatively short time even when the incandescent lamps are operated through a transformer at a reduced voltage in order to gain a longer lamp life. This basic problem has previously been recognized inter alia by Walton et al in U.S. Pat. Nos. 4,782,429 and 5,012,157, these patents describing lamps useful for replacement of incandescent lamps in exit signage and the like by the provision of low voltage, heated filament lamps which are intended to operate for a longer period of time than the incandescent lamps which have long been used in exit signage.
While illuminated signs have been proposed for a number of years with light emitting diodes as the illumination source, the use of light emitting diodes in original equipment exit signs has been expensive and, as importantly, the prior reliability of light emitting diodes has caused these illuminated signs to be less than practical. In recent years, light emitting diodes have improved to the point that such diodes have come into use as the original illumination source in certain categories of exit signage and the like. While the expense inherent in the use of light emitting diodes as the illumination source in an exit sign is still substantially greater than exit signs having incandescent or fluorescent illumination sources, the ability of the LED illumination source to provide extremely long life and energy efficiency has caused acceptance of LED-illuminated exit signs in the marketplace. Coincidental with the acceptance of LED-illuminated original equipment exit signage has been the provision of LED illumination sources in association with standard incandescent lamp bases to the end that the "solid state" or LED lamp so provided can be used in existing standard AC or DC lamp sockets. Ray, in U.S. Pat. No. 4,211,955, disposes light emitting diodes within a standard incandescent light bulb and lamp base. The solid state lamp thus provided by Ray can be used to replace any standard incandescent light bulb and inserted into existing standard AC or DC lamp sockets depending upon the standard base chosen. The circuitry of Ray includes an integrated circuit chip as a major portion of the operating circuit described in this patent. Uchida, in U.S. Pat. No. 4,727,289, disposes light emitting diodes on a printed circuitboard arranged inside a glass bulb having a base intended to be received into a standard socket. Power resistors used by Uchida are part of circuitry intended to provide a "high voltage" lamp but which also provides dissipative characteristics due to the wire-wound construction of the power resistors. In U.S. Pat. No. 4,939,427, Menard et al describe a lamp utilizing light emitting diodes as the illumination source, the lamp being insertable through a conventional base into a socket powered by an AC input. Other patents exist which utilize light emitting diodes as an illumination source whether with or without a covering envelope but with a base form intended to be received within a socket through which the LED illumination source is powered. Such patents include U.S. Pat. No. 4,290,095 to Schmidt, U.S. Pat. No. 4,630,183 to Fujita; U.S. Pat. No. 5,160,200 to Cheselske and U.S. Pat. No. 3,795,830 to Richardson. The disclosures of all of the patents mentioned hereinabove are incorporated hereinto by reference.
In view of the prior art described above, certain significant benefits are to be derived from a series connection of a significant number of light emitting diodes in arrangements proposed according to the present invention. The present circuit arrangements include attendant reduction of losses and stresses on circuit components. The invention thus provides LED illumination sources contained within a conventional glass or plastic bulb-like envelope of standard shape and dimensions and associated with common lamp bases such as intermediate, medium candelabra and double contact bayonet inter alia, such that the LED lamp of the invention can be retrofitted into existing exit signage which use incandescent lamps fitted with the corresponding lamp base.